i've got my eyes on a DIY project which requires to be powered by a dual rail PSU.Just wondering if it is possible to buy a wall wart, or similar externnal PSU, affordably with dual rails...i see computer PSU's can come with dual rails, but not so sure it's suitable for this purpose.

Top of the list you'll find a +/-16V supply that also provides a 48V for phantom power. There are a range of other supply kits to suit almost every need.

Bear in mind that without a centre tapped transformer, making a +/- supply from a wall wart provides a reference ground rather than an earth. Suppose you get a 24V DC wall wart - a couple of 100K resistors and 12V zener diodes are all you need to create a half volts supply rail, add in some bypass caps as needed. Call this half volts rail 0V and then the wall wart positive is +12V and the wall wart negative is -12V. Lo and behold, dual supply!

The danger is forgetting that the 0V in this arrangement is actually at 12V to the rest of the world - it is NOT earthed - so you do NOT connect it to your case, for example, and you need to have a transformer or other scheme to send any signal out into the real world.